When transmitting data on a shared bandwidth network such as the IP network, it is known that the communication quality changes in time due to what is called cross traffic, which is traffic transmitted and received by other services and applications. When there is a wireless link on the communication path, the change in the link quality due to the cross traffic and external noise is particularly prominent compared to the wired link.
In the communication using wireless links, the transmitted signal wave attenuates along the transmission distance. Thus, the direct communication can only be performed with reception nodes within a predetermined range. This range differs depending on the difference of the frequency band or modulation method in the wireless communication layer. For example, according to 802.11a, which is a 5.2 GHz wireless communication method standardized by the Institute of Electrical and Electronic Engineers (IEEE), the range is approximately 70 m under a condition with a transmission power of 16 dBm, a reception sensitivity of −69 dBm, and a transmission rate of 54 Mbps. Communication with the reception nodes outside the range is performed by a bucket-brigade relay of data by another node within the area. Such a communication is referred to as a multi-hop communication.
In multi-hop communication, it is usually preferable that the data transmitted from the transmission node reaches the reception node through a shortest route. In order to do so, it is necessary for each relay node in the paths to relay data to the most suitable neighboring relay node. The process for determining the most suitable path for relaying the data from the transmission node to the reception node is referred to as routing and a protocol for the nodes when routing is referred to as the routing protocol.
Various routing protocols according to the purpose has been proposed for wireless networks, and other than selecting the shortest path, there is a protocol for choosing the path with the best transmission status as the entire path. The index for evaluating the path for a good path is referred to as the routing metrics, and there are various evaluation standards depending on the routing protocol.
The routing protocol using a single data transmission path between the transmission node and the reception node is referred to as the single path method. Optimized Link State Routing (OLSR) and Ad hoc On-Demand Distance Vector (AODV) are major single-path methods in wireless ad hoc network.
In the case of the single path method, when the quality of the link in the wireless link on the path reduces as described above, the path is cut off, stopping the data transmission, and the process for finding another path starts. The communication resumes using the newly found path. However, there is a big delay for recovering the path, and the data transmitted when the communication stopped is lost.
There is a method in which single or multiple secondary paths are provided in advance, in addition to the path used for transmission (primary path), in order to smoothly switch the path. The method is referred to as multi-path method, or multi-path routing. Patent Literature 1 discloses an example of conventional technology of multi-path routing, in which the next hop node corresponding to each path is recorded on the routing table together with the evaluation value of the path, and a path with the highest evaluation value is selected.
FIG. 12 is a structural diagram illustrating an example of the network according to the conventional technology.
When the data transmitted from the transmission node 401 to the reception node 406 is transmitted through the relay node 402, and the relay node 404, the path is the primary path. The path to the reception node 406 from the relay node 402 through the relay node 403 and the path to the reception node 406 from the relay node 402 through the relay node 405 are secondary paths provided as extra paths. The secondary paths are used as an alternative when the data transmission using the primary path fails. The routing information of the primary path and the secondary paths is recorded and managed on the routing table, according to the multi-path routing protocol.
FIG. 13 illustrates an example of the routing table managed on the relay node 402. The paths found based on the routing protocols are recorded on the routing table provided in each relay node. In the routing table, the metrics for each path is managed, in addition to the address of the relay node of next hop that is a downstream-transfer destination. Thus, when the process is in condition for switching from the primary path to the secondary path, a secondary path with the best metrics value among the secondary paths is selected as the primary path.
The number of relay node which is used for relaying the data from the relay node 402 to the reception node 406 is an example of the metrics, and this index is referred to as the hop count. Since the secondary path is found in advance and recorded on the routing table, this method has less recovery delay due to the path failure and less data loss upon switching the path, compared to the single path method.
[Citation List]
    [Patent Literature 1] Japanese Patent No. 4060316    [PTL 1]    [Non Patent Literature 1] IETF RFC3651 Ad hoc On-Demand Distance Vector (AODV) Routing    [Non Patent Literature 2] IETF RFC3626 Optimized Link State Routing Protocol (OLSR)    K. Leibnitz, N. Wakamiya, and M. Murata, “Resilient multi-path routing based on a biological attractor selection scheme” in The Second International Workshop on Biologically Inspired Approaches to Advanced Information Technology (BioAdit 2006), Osaka, Japan, January 2006.